Radio Frequency Identification (RFID) is an automatic identification technology, relying on storing and remotely retrieving data, using devices called RFID tags or transponders. The RFID system consists of two major components—an interrogator/reader and a data carrier, such as a data tag or data label. These components work together to provide the end user with a non-contact solution to uniquely identify people, animals or objects. RFID data tags have recently become widely used for tracking objects, articles and products. Unlike optic barcode systems, RFID does not require line-of-sight and greatly reduces costs associated with the reading of each barcode tag.
An RFID tag is an object that can be attached to or incorporated into a product, animal, or person for the purpose of identification, using radio waves. Generally, RF tags can be active (utilizing an internal energy source incorporated with the tag, e.g., a battery) or passive, functioning by using the energy of an external interrogation signal and dependent on energy supplied from a tag reader or an external device.
An active RF tag typically includes an antenna attached to a resonance circuit, which is energized by the received interrogation signal and which, when energized, excites the antenna to transmit a response radio frequency signal. Passive tags do not include an energy source, but only respond to existing radiation by retransmitting, reflecting, or scattering, and typically do not include active elements.
A passive RFID tag that does not depend on a silicon microchip is usually referred to as a chipless tag. Some chipless tags use plastic or conductive polymers instead of silicon-based microchips. Other chipless tags use materials that reflect back a portion of the radio waves beamed at them. They might be implemented also by using etching or conductive ink printing technologies and can be printed directly on articles as etched or screen printed metal-based antennas.
RFID tags can be placed on or in an article that might be used in retail or for large manufacturing, warehousing and distribution facilities. For instance, in the pharmacological industry these tags can be used for identifying the manufacturer or company entity, the drug class, product's name, and also serial number. Likewise, the tags can carry information such as the drug's dates of manufacture and expiry, batch number, price and even destination data.
U.S. Pat. No. 6,997,388 assigned to the Applicant of the present invention describes a radio frequency (RF) data tag. The RF data tag comprises at least one diffraction element that has a dimension of the order of a wavelength of RF radiation and is responsive to RF irradiation, the response produced by the diffraction elements in the data tag being indicative of machine-readable data carried by the data carrier. U.S. Pat. No. 6,997,388 describes a diffraction pattern originated from the diffraction elements that define a symbol in a data that symbolize a data marking information code-language. The diffraction elements are made of materials having a specific RF diffraction absorption, reflection or scattering properties different from that of the substrate material. The reflective material can, for example, be a conductive ink, which is printed on a substrate, which, on RF radiation, causes the diffraction pattern.
European Patent Application No. 1 065 623 describes microwave readable barcodes and microwave barcoding systems. Microwave readable barcodes have conductive bars that selectively resonate with incoming microwave signals. Conductive bars can be made from conductive ink or from a conductive foil. Barcode information can be encoded using conductive bars of different lengths, different angles, or different positions. Microwave readable barcode systems include a barcode made from conductive bars, a transmitter that radiates a microwave signal onto the barcode, and a sensor that senses the effect of the conductive bars on the microwave signal. Sensors can sense the attenuation or the non-attenuation of the microwave signal by the conductive bars, and/or the scattering or the non-scattering of the microwave signal by those bars.
One of the techniques used for RFID tag identification is RADAR (radio direction and ranging) that is widely used for detection of objects (targets) navigation and ranging. As in RFID systems, radar also uses a transmitter to illuminate an object and a receiver to detect its existence or position (or both).
For example, U.S. Pat. No. 6,529,154 to Schramm. Jr. et al. describes a method and apparatus for sensing two-dimensional identification marks provided on a substrate or embedded within a substrate below a surface of the substrate. Micropower impulse radar is used to transmit a high rise time, short duration pulse to a focused radar target area of the substrate having two dimensional identification marks. The method includes listening for radar echoes returned from the identification marks during a short listening period window occurring a predetermined time after transmission of the radar pulse. If radar echoes are detected, an image processing step is carried out. If no radar echoes are detected, the method further includes sequentially transmitting further high rise time, short duration pulses, and listening for radar echoes from each of said further pulses after different elapsed times for each of the further pulses until radar echoes are detected. When radar echoes are detected, data based on the detected echoes is processed to produce an image of the identification marks.
U.S. Pat. Appl. Publication No. 2005/0280539 to Pettus describes a system and method for encoding and decoding information by use of radio frequency antennas. The system includes one or more interrogator devices and RFID data tags. The RFID data tags include a plurality of antenna elements, which are formed on a substrate or directly on an object. The antenna elements are oriented and have dimensions to provide polarization and phase information, whereby this information represents the encoded information on the RFID tag. The interrogator device scans an area and uses radar imaging technology to create an image of a scanned area. The device receives re-radiated RF signals from the antenna elements on the data tags, whereby the data tags are preferably represented on the image. The re-radiated RF signals preferably include polarization and phase information of each antenna element, whereby the information is utilized using radar signal imaging algorithms to decode the information on the RF data tag.
It should be noted that encoding tags with information by using phase and polarization can be impractical and expensive. The phase of the reflection by a given element is dependent on the distance from the transmitting antenna to that element and back to the receiving antenna. For example, at the operating frequency of 60 GHz (i.e., the wavelength of 5 mm), a slight bending of the tag, by say 0.625 mm, can produce a phase shift of 90 degrees, which makes the phase information totally useless. Thus, it appears that phase information is too sensitive to be relied on.
Moreover, polarimetric techniques described in US 2005/0280539 involve transmission and reception in both polarizations. Such methods can provide characterization of the illuminated tags, but are hard and expensive to implement. In particular, polarimetric measurements are sensitive and hard to calibrate. Likewise, reception of the co-polarized radiation makes the interrogator susceptible to desensitization due to strong reflections by neighboring objects.
A Synthetic Aperture Radar (SAR) technique is known, which performs sophisticated post-processing of radar data and is used to produce a narrow effective beam, thereby significantly increasing the system detection capability and resolution. Synthetic Aperture Radar (SAR) images can be obtained by processing radar scattering data collected over a range of angles and frequencies (see, for example, D. L. Mensa, High Resolution Radar Cross Section Imaging (2nd ed.), Boston: Artech House, 1991; M. Soumekh, Synthetic Aperture Radar Signal Processing; New York: John Wiley & Sons, 1999).
In SAR, data collection is performed with a radar moving across the line-of-sight, while the target is stationary. On the other hand, an Inverse SAR (ISAR) refers to the case when the target is moving (usually rotated), while the radar is stationary. Radars can operate either in a continuous wave (CW) mode or pulsed mode, and employ one or more transmitting and receiving antennas.
The motion of the transmitting and/or receiving antenna may be provided mechanically or simulated by the antennas' array electronic switching. The optimum geometric resolution that can be provided with SAR is determined by centre frequency and bandwidth of the transmitted signal and the aperture angle, over which the antenna, along the straight path, illuminates the target area.
Synthetic Aperture Radar images can be obtained by processing radar scattering data collected over a range of angles and frequencies. Under far-field conditions, SAR and ISAR signal processing is conventionally reduced to a multidimensional Fourier transform, which is performed by the Fast Fourier Transform (FFT) algorithm (see, for example, D. L. Mensa, High Resolution Radar Cross Section Imaging (2nd ed.), Boston: Artech House, 1991, Chapter 5, pp. 139-200; M. Soumekh, Synthetic Aperture Radar Signal Processing; New York: John Wiley & Sons, 1999, Chapter 4, pp. 176-212). Moreover, fast algorithms for near-field processing are also known (see, for example, A. Boag, “A Fast Multilevel Domain Decomposition Algorithm for Radar Imaging,” IEEE Trans. Antennas and Propagation, vol. 49, no. 4, pp. 666-671, April 2001).